Thermotropic liquid crystals are generally crystalline compounds with significant anisotropy in shape. That is, at the molecular level, they are characterized by a rod-like or disc like structure. When heated they typically melt in a stepwise manner, exhibiting one or more thermal transitions from a crystal to a final isotropic phase. The intermediate phases, known as mesophases, can include several types of smectic phases wherein the molecules are generally confined to layers; and a nematic phase wherein the molecules are aligned parallel to one another with no long range positional order. The liquid crystal phase can be achieved in a heating cycle, or can be arrived at in cooling from an isotropic phase. The structure of liquid crystals in general, and twisted nematic liquid crystals in particular, is further discussed in “The Physics of Liquid Crystals”, de Gennes and Prost, Oxford University Press, 1995.
An important variant of the nematic phase is one wherein a chiral moiety is present, referred to as a twisted nematic or cholesteric phase. In this case, the molecules are parallel to each other as in the nematic phase, but the director of molecules (the average direction of the rodlike molecules) changes direction through the thickness of a layer to provide a helical packing of the nematic molecules. The pitch of the helix is perpendicular to the long axes of the molecules. This helical packing of anisotropic molecules leads to important and characteristic optical properties of twisted nematic phases including circular dichroism, a high degree of rotary power; and the selective reflection of light, including ultraviolet, visible, and near-IR light. Reflection in the visible region leads to brilliantly colored layers. The sense of the helix can either be right-handed or left-handed, and the rotational sense is an important characteristic of the material. The chiral moiety either may be present in the liquid crystalline molecule itself, for instance, as in a cholesteryl ester, or can be added to the nematic phase as a dopant, with induction of the cholesteric phase. This phenomenon is further discussed in sources such as Bassler and Labes, J. Chem. Phys., 52, 631 (1970).
There has been interest in preparing stable polymer layers exhibiting nematic and/or cholesteric optical properties. One approach has been to synthesize monofunctional and/or polyfunctional reactive monomers that exhibit a nematic or cholesteric phase upon melting, formulate a low melting liquid crystal composition, and polymerize the liquid crystal composition in its nematic or cholesteric phase to provide a polymer network exhibiting stable optical properties of the nematic or cholesteric phase. Use of cholesteric monomers alone, as disclosed in U.S. Pat. No. 4,637,896 for example, provides cholesteric layers with the desired optical properties, but the polymer layers possess relatively weak mechanical properties.
A need thus remains for liquid crystal compositions that have broad thermal windows, low melting points and good phase stability against crystallization, and that are easy to prepare and can be tuned to give desired properties.